Ads For Old Posts
The idea behind this plugin was simple; we wanted the ability to show ads on blog posts, but didn’t want those ads to display for our most loyal readers. Instead, we wanted to display the ads only if the post was past a certain age.
With Ads For Old Posts you can add in an advertisement (like google adsense) and have it display at the top or bottom of every blog post as long as it was published more than a certain number of days ago (30 days by default). The plugin also allows for some styling of the ad block so you can make it stand out or blend in as you see fit.
Simple solution, free plugin.